During the last decade, considerable effort has been expended to develop a clinically reliable test for use in counseling and prenatal detection of cystic fibrosis (CF) genotypes. Many tests, such as leukocyte degranulation, sodium reabsorption inhibition in rat parotids, and ciliary dyskinesia, have been proposed as bioassay procedures for the CF factor, which is present in serum and possibly most cells, of cystic fibrosis homozygotes and heterozygotes. The majority of these procedures, however, have not been reproducible from lab to lab, have not been accurate at a high level of statistical certainty, and cannot discern heterozygotes from homozygotes. The bioassay procedure, with which the authors have considerable experience, utilizes direct current measurements on rat jejunum to achieve a 90% accuracy in discerning between normal and CF genotypes; however, the assay is unable to distinguish between different CF genotypes, is extremely time-consuming, and does not offer over a 90% reliability. The authors propose to substitute computer-controlled alternating current membrane impedance measurements for the direct current determinations previously used. It is anticipated that this action, through increased measurement speed, will improve the accuracy, and along with increased amounts and types of data which alternating impedance measurements glean, will improve the clinical utility of the test by enabling separation of homo- and heterozygote genotypes.